The present invention relates to the diagnosis of the syndrome of sleep apnea and more particularly, cardiac pacemakers able to detect sleep apnea and respond to the detection with electrostimulation.
The syndrome of sleep apnea (xe2x80x9cSASxe2x80x9d), more precisely the syndrome of obstructive and non central sleep apnea (xe2x80x9cSOASxe2x80x9d) is an affliction having generally as its origin an obstruction of the respiratory tracts. It is likely to involve a certain number of disorders such as painful and/or insufficient breathing, an abnormal heartbeat, and hypertension. Various treatments of SAS have been proposed, including treatments involving surgery, medication, and maintenance of a positive pressure in the respiratory tract by means of a facial mask worn during sleep.
One technique, as discussed in EP-A-0 702 979 (to Medtronic) proposes to treat SAS by electrostimulation. This document describes an implanted pulse generator, controlled by a sensor, which may be a dynamic pressure sensor or a sensor of intrathoracic impedance, making it possible to follow (monitor) the patient""s respiration rate and thus to detect the occurrence of an apnea. When an apnea is detected, the generator delivers a salvo (sequence) of pulses to a stimulation electrode implanted in the muscles controlling the patient""s airway. This technique is not, however, in practice, completely satisfactory. This is because the stimulation which is systematically started in the event of an increase in the intratracheal pressure, whatever the cause of this increase in pressure, and whether it is due to an SAS or not, will include inappropriate stimulations.
Pacemakers having a cardiac stimulation or pacing rate which is responsive to a detected physiological or physical parameter of the patient are known. Generally, as the measured parameter increases, it reflects an increasing level of activity of the patient (e.g., exercise), and the stimulation frequency increases so that the pacing rate is controlled to simulate the action of a normal heart. Once such style of pacing device measures the patient""s so-called minute ventilation (minute volume) based on a transthoracic or intrathoracic impedance measurement. An earlier style of such a pacing device measured the respiration rate, but this parameter is generally believed to be less useful as a physiological parameter because it does not represent the patient""s metabolic demand (also referred to as the cardiac output requirements) during phases of increased patient activity.
In the case of cardiac pacemakers, all these systems operate to increase the frequency of stimulation pulses when one detects an increasing activity of the patient wearing the device (i.e., the patient in which the device is implanted or on which the device is carried), and to decrease this frequency to a base value in the case of a diminution of activity, particularly during phases of rest of the patient.
EP-A-0 493 222 describes a process of correlation between, on the one hand, the two extreme values FCbase and Xmax of the range of the stimulation frequency and, on the other hand, value Xbase and Xmax, which are respectively the rest value and the value of maximal activity, calculated from information collected by the sensor measuring the detected physiological or physical parameter (also called an xe2x80x9censlavement sensor.xe2x80x9d) This process of correlation is known under the name of xe2x80x9cautomatic calibration of the enslavementxe2x80x9d, and the document describes a process to determine the value of Xbase in the case of the utilization of the minute-ventilation as the parameter of enslavement. The value of the minute-ventilation at rest is then called MVrest. This last value is obtained by the calculation of an average value during an interval on the order of 24 hours, including, therefore, periods of activity as well as periods of sleep of the wearer of the device.
It has been observed and recognized that, during phases of sleep, the values of MVrest can be more than 50% below the values of this same parameter recorded during periods when the patient is awake (i.e., conscious) and active.
Many parameters, including, but not limited to, the minute ventilation, the respiratory frequency, the saturation of oxygen in the blood, the temperature, or the acceleration have been acceptably used as parameters of enslavement for control functions. In particular, these parameters have been used in the case of cardiac pacemakers, to vary the instantaneous frequency of the cardiac stimulation according to the measured or calculated parameter.
The utilization of one or more, and more particularly several, sensors, is at the expense of an incremental energy consumption. This is due to the additional hardware circuits, the increase of which is directly associated to the enslavement parameter transducer(s) (power supply, injection of current (as in the case of minute ventilation and other sensors), production and analysis of the signal, etc.), as well as the software used to process the sensor produced signals. It is generally realized that the microprocessors or specific circuits executing the software or logic functions are typically large, energy-consuming components when they execute algorithms to process data and make decisions.
As used herein, the terms xe2x80x9censlavementxe2x80x9d and xe2x80x9censlavedxe2x80x9d mean the control function has a determined result or output that varies as a function of the monitored parameter. The functional relationship may be linear, non-linear, defined by an algorithm or a look-up table, and may be predetermined or self-adjusting.
In the case of cardiac pacemakers, all these enslavement systems compete to increase the stimulation pulse frequency when one detects an increasing activity of the patient, and to decrease the stimulation pulse frequency to a base or minimum frequency in case of a diminution of activity, and particularly during phases of rest of the patient.
It is, therefore, an object of the invention to propose a device for the treatment of SAS by electrostimulation.
Broadly speaking, the present invention concerns analyzing the metabolic and functional state of the patient, for applying, selectively, a stimulation for the treatment of SAS only during the phases of patient activity where an SAS is really likely to appear, and otherwise inhibiting any SAS stimulation.
One aspect of the invention is directed to a device which is an active implantable medical pacemaker device allowing for the treatment by increased cardiac electrostimulation of the sleep apnea syndrome in a patient i.e., including: means for measuring the respiratory activity of the patient; means for analyzing and determining an occurrence of an apnea in response to the measured respiratory signal; and means for delivering an SAS stimulation, controlled by the analyzing means, so as to apply selectively to the patient an increased cardiac stimulation rate in the event of a detection of an apnea. The SAS stimulation means is preferably a circuit which delivers SAS stimulation by increasing cardiac stimulation rate, and the respiratory activity measurement means may be a circuit which includes a minute ventilation sensor or a sensor which detects the oxygen saturation of the blood.
According to a preferred embodiment of the present invention, this device also includes means for determining a cardiac rate of the patient, including a rate in the absence of a determined apnea, means for determining a state of activity of the patient, this state being likely to take, according to predetermined criteria, a value representative of a state of sleep (also referred to as a rest phase) of the patient, such that the SAS stimulation means is triggerable only during a determined phase of sleep and otherwise is inhibited.
According to other various advantageous characteristics of the invention, the analyzing means detects an occurrence of a syndrome of sleep apnea when the number of apnea occurrences detected during a given period of time exceeds a predetermined threshold. In another embodiment, the determining means optionally determines a state of activity by analyzing the signal delivered by the means for measuring the respiratory activity of the patient, and/or by a separate auxiliary measurement means.